Well there was one expedition called the Transglobe expedition, but the flat earths have issues with it and claim shenanigans. I am surprised it hasn't been attempted more than once.. People love doing crazy stuff like climbing Everest.
I would agree with you in regards to everything else you said, if there is a God it used the laws of nature. To me.. God kind of is the laws of nature and nature itself not an actual separate being, so.. I'm sorta with you there and in regards to the other stuff you said.
But umm.. Flat Earthers generally don't believe in GPS and satellites, and I'm not sure how they would respond to the rest of your argument. in regards to the trip being made with position of the sun and stars and such. I think they believe the earth is stationary so all the other stuff moves and we don't, theoretically you should still be able to find your way based on the stars to some extent though in a flat earth model. Not totally sure how it works though..